Aqueous polymer dispersions or aqueous polymer lattices based on vinyl acetate and ethylene are conventionally prepared by the suspension polymerization process, more particularly by the emulsion polymerization process. In their preparation, the polymerization is not taken to complete conversion, for economic reasons. There remain residual monomer fractions, which are removed by means of chemical methods such as postpolymerization and/or of physical methods such as stripping methods, in which a stripping gas is passed through the aqueous dispersion. Environmental and economic reasons dictate that the attendant waste gases and wastewaters shall be purified and that the monomer fraction removed in the course of degassing and, optionally, stripping, more particularly the vinyl acetate fraction, shall be recovered and, where appropriate, re-used in the polymerization.
Known from DE 4425918 C2 is a method for recovering unreacted vinyl acetate monomer (VAM) after polymerization reactions from the steam that escapes during stripping, and in this method the vinyl acetate is isolated, by extraction in an extraction column, from the process water from a water ring pump which takes off the gas phase during stripping of the dispersion.
While this method does recover VAM from the steam that escapes during stripping, the gas stream taken off by the water ring pump, and also the wastewaters obtained in the extraction column, nevertheless still contain residual quantities of VAM.
WO 2007/074075 A1 describes the removal of residual vinyl ester and ethylene monomers, after the end of polymerization, by means of a multistage fractional, very low-temperature condensation. The purification of wastewaters obtained during the polymerization is not described.